This invention relates to apparatus for generating digital signals. More particularly, it is concerned with apparatus for producing digital signalling codes for use in digital communication systems.
In telephone communication systems a variety of signalling codes are employed in the operation of the system. Certain of these signalling codes must be audible tones to the telephone subscriber. In digital communication systems the signalling codes as well as the communication data are transmitted in digital format. It has been customary to provide audible signalling tones by employing an analog signal generator, converting the analog signals to digital signals for transmission, and then re-converting the digital signals to audible signalling tones.
Apparatus for use in a pulse code modulation digital system to provide digital signalling codes which when converted to analog signals provide audible tones to the subscriber is described in an article entitled "Multifrequency (MF) Tone-Generating System for a Pulse-Code Modulation (PCM) Digital Exchange" by Satyan G. Pitroda published in the IEEE Transactions on Communication Technology, Vol. COM-19, No. 5, Oct. 1971, pages 588-596. This article describes a technique of presynthesizing tone signals as patterns of digital bits which are stored in read-only memories. These digital tone signals are transmitted as PCM digital signals and are converted to analog signals to provide audible tones to the subscriber.
In addition to audible tones many other signalling tones in the form of digital codewords are employed in the operation of digital telephone communication systems. Some of the signalling codes may be combinations of codewords which automatically cause operations to be performed in the system together with tone signals for the benefit of subscribers. The digital signalling codes must be generated and made available to the channels of the system. Presently, as described in the aforementioned article, the signalling codes are continuously generated, and the injection system for outpulsing the signals from the generators to the communication channels includes two sets of crosspoints. Appropriate address information is stored and is utilized to close appropriate crosspoints at the proper times to inject the selected signalling codes in the proper channel time slots.